There are many known power assist steering systems for automotive vehicles. Some provide steering assist by using hydraulic power and others by using electric power.
Electric assist steering systems include an electric motor drivably connected to the steerable vehicle wheels. When energized, the electric motor assists the steering movement of the steerable wheels. The electric assist motor is controlled in response to steering torque applied to the steering wheel.
Known electric steering systems typically include a D.C. permanent magnet electric assist motor electrically energized through an H-bridge drive circuit. It is desirable to, instead, use a variable reluctance motor for an electric assist steering system because of its smaller size and larger torque-to-inertia ratio. Proper commutation of a variable reluctance motor requires a "knowledge" of the motor's rotor position relative to the stator.
Rotor position sensors for electric motors are known in the art. For example, U.S. Pat. No. 2,650,352 to Childs is directed to a system for measuring the motion between two relatively movable members (i.e., rotor and stator). The system comprises a rotor and a stator both made of a non-magnetic material to avoid magnetic coupling errors that arise in any ferromagnetic structure. In one embodiment, a conductor is threaded back and forth through accurately spaced slots around the periphery of the rotor, and likewise for the stator. The conductor patterns on both the rotor and stator form a plurality of series connected conductor bars disposed transverse to the direction of movement of the rotor. A radio frequency current is applied to the terminals of the conductor on the stator. The radio frequency current in the stator conductor induces a signal in the rotor conductor. The magnitude of the induced signal in the rotor conductor is a function of the relative position between the rotor and the stator. In this way, the system measures the relative position between the rotor and the stator.